In the end it turned out to be a very trivial problem - it seems the usb socket of the kobo was a bit misaligned or something like that because most of the usb cables couldnt fit perfectly (including the one I have used by now) . But one cable worked and now my reader is updated and working even better than before.
I thank you for the provided support and sorry for the wasted time. At least I experimented a bit with the technical stuff and it was a pleasant experience
